Rubber composition and crosslinked rubber molded product

ABSTRACT

An object of the present disclosure is to provide a rubber composition from which a crosslinked rubber molded product having good softness and excellent resilience performance can be obtained. The present disclosure provides a rubber composition containing (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) a crosslinking initiator, and (d) a benzothiazole derivative, and further comprising (e) a metal compound when (b) the co-crosslinking agent consists of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, wherein (d) the benzothiazole derivative is a compound represented by the formula (1) and/or a compound represented by the formula (2). 
     
       
         
         
             
             
         
       
         
         
           
             [R 1  represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group having 4 to 14 carbon atoms, or a metal atom, 
             R 2  to R 5  are identical to or different from each other, and represent an electron-withdrawing group or a hydrogen atom, and at least one of R 3  to R 5  is an electron-withdrawing group.]

FIELD OF THE INVENTION

The present disclosure relates to a rubber composition, and particularlyrelates to a rubber composition from which a crosslinked rubber moldedproduct having excellent resilience can be obtained.

DESCRIPTION OF THE RELATED ART

As a method for increasing a flight distance of a golf ball on drivershots, for example, there is a method of using a core having highresilience. Enhancing the resilience of the core increases the initialspeed of the golf ball, thereby increasing the flight distance of thegolf ball. Here, a rubber composition generally contains a base rubber,a co-crosslinking agent and a crosslinking initiator. In addition, it isknown that the rubber composition may further contain a vulcanizationaccelerator.

For example, JP 2005-000647 A discloses a golf ball comprising acomposition containing an unsaturated polymer, a crosslinking agent, apeptizer which is a non-metal salt of an organic sulfur compound, and anaccelerator selected from the group consisting of2-mercaptobenzothiazole and a salt of 2-mercaptobenzothiazole.

In addition, JP 2005-000657 A discloses a golf ball comprising acomposition containing an unsaturated polymer, a crosslinking agent, apeptizer, and a vulcanization accelerator, wherein the vulcanizationaccelerator is selected from the group consisting of2-mercaptobenzothiazole and a salt of 2-mercaptobenzothiazole.

SUMMARY OF THE INVENTION

Although various rubber compositions for enhancing the resilience havebeen proposed, there is still room for improvement in the resilienceperformance. The present disclosure has been achieved in view of theabove circumstances, and an object of the present disclosure is toprovide a rubber composition from which a crosslinked rubber moldedproduct having good softness and excellent resilience performance can beobtained.

The present disclosure that has solved the above problems provides arubber composition containing (a) a base rubber, (b) an α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereofas a co-crosslinking agent, (c) a crosslinking initiator, and (d) abenzothiazole derivative, and further comprising (e) a metal compoundwhen (b) the co-crosslinking agent consists of the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms, wherein (d) thebenzothiazole derivative includes a compound represented by the formula(1) and/or a compound represented by the formula (2).

[R¹ represents a hydrogen atom, an alkyl group having 1 to 8 carbonatoms, an aryl group having 4 to 14 carbon atoms, or a metal atom,

R² to R⁵ are identical to or different from each other, and represent anelectron-withdrawing group or a hydrogen atom, and at least one of R³ toR⁵ is an electron-withdrawing group.]

If the rubber composition according to the present disclosure is used, acrosslinked rubber molded product having good softness and excellentresilience performance can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a FIGURE showing a relationship between the coefficient ofrestitution and the compression deformation amount of the crosslinkedrubber composition.

DESCRIPTION OF THE PREFERRED EMBODIMENT [Rubber Composition]

The present disclosure provides a rubber composition containing (a) abase rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms and/or a metal salt thereof as a co-crosslinking agent, (c) acrosslinking initiator, and (d) a benzothiazole derivative, and furthercomprising (e) a metal compound when (b) the co-crosslinking agentconsists of the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms, wherein (d) the benzothiazole derivative includes a compoundrepresented by the formula (1) and/or a compound represented by theformula (2) which will be described later.

(d) Benzothiazole Derivative

(d) The benzothiazole derivative used in the present disclosure will beexplained. Examples of (d) the benzothiazole derivative includes thecompound represented by the formula (1), and the compound represented bythe formula (2). (d) The benzothiazole derivative may be used solely, ortwo or more of them may be used in combination. Blending (d) thebenzothiazole derivative in the rubber composition accelerates thecrosslinking reaction and densely forms the crosslinking, and thus theresilience is improved without impairing the softness.

[R¹ represents a hydrogen atom, an alkyl group having 1 to 8 carbonatoms, an aryl group having 4 to 14 carbon atoms, or a metal atom,

R² to R⁵ are identical to or different from each other, and represent anelectron-withdrawing group or a hydrogen atom, and at least one of R³ toR⁵ is an electron-withdrawing group.] Herein, at least one of R³ to R⁵means any one of R³, R⁴, or R⁵, any combination of two of R³, R⁴, andR⁵, or all of R³, R⁴, and R⁵.

Examples of the alkyl group having 1 to 8 carbon atoms represented by R¹in the formula (1) or the formula (2) include a linear alkyl group, abranched alkyl group, and a cyclic alkyl group. The alkyl grouppreferably has 1 or more carbon atoms, and preferably has 6 or lesscarbon atoms, more preferably has 4 or less carbon atoms.

Examples of the linear alkyl group include a methyl group, an ethylgroup, a n-propyl group, a n-butyl group, and a n-pentyl group.

Examples of the branched alkyl group include an isopropyl group, anisobutyl group, a s-butyl group, a t-butyl group, and an isopentylgroup.

Examples of the cyclic alkyl group include a cyclopropyl group, acyclobutyl group, a cyclopentyl group, and a cyclohexyl group.

The aryl group having 4 to 14 carbon atoms represented by R¹ preferablyhas 12 or less carbon atoms, more preferably has 10 or less carbonatoms. Examples of the aryl group include a phenyl group, and a naphthylgroup.

Examples of the metal atom represented by R¹ include sodium, potassium,lithium, magnesium, calcium, zinc, barium, and cadmium.

R¹ is preferably the hydrogen atom or the alkyl group having 1 to 8carbon atoms, more preferably the hydrogen atom.

The electron-withdrawing group represented by R² to R⁵ is a substituentgroup whose force withdrawing electron from the carbon atom which thesubstituent group is bonding is greater than that of a hydrogen atom.Examples of the electron-withdrawing group represented by R² to R⁵include a halogen group, a perfluoroalkyl group, a halogenated alkylgroup, an alkylcarbonyl group, an alkoxycarbonyl group, apentafluorosulfanyl group (—SF₅), a nitro group (—NO₂), a cyano group(—CN), a carboxy group (—COOH), an aldehyde group (—CHO), a sulfanylgroup (—SH), a sulfo group (—SO₃H), an alkylsulfonyl group, analkoxysulfonyl group, and a perfluoroalkylsulfonyl group.

Examples of the halogen group include a fluoro group (—F), a chlorogroup (—Cl), and a bromo group (—Br).

Examples of the perfluoroalkyl group include a trifluoromethyl group(—CF₃), a pentafluoroethyl group (—C₂F₅), and a heptafluoropropyl group(—C₃F₇).

Examples of the halogenated alkyl group include a trichloromethyl group(—CC₃), and a monochloromethyl group (—CH₂Cl).

Examples of the alkylcarbonyl group include an acetyl group (—COCH₃),and a propionyl group (—COC₂H₅).

Examples of the alkoxycarbonyl group include a methoxycarbonyl group(—COOCH₃), and an ethoxycarbonyl group (—COOC₂H₅).

Examples of the alkylsulfonyl group include a methylsulfonyl group(—SO₂CH₃), and an ethylsulfonyl group (—SO₂C₂H₅).

Examples of the alkoxysulfonyl group include a methoxysulfonyl group(—SO₂OCH₃), and an ethoxysulfonyl group (—SO₂OC₂H₅).

Examples of the perfluoroalkylsulfonyl group include atrifluoromethylsulfonyl group (—SO₂CF₃), and a pentafluoroethylsulfonylgroup (—SO₂C₂F₅).

The electron-withdrawing group represented by R² to R⁵ is preferably onemember selected from the group consisting of the halogen group, theperfluoroalkyl group, and the pentafluorosulfanyl group.

(d) The benzothiazole derivative preferably includes a compoundrepresented by the formula (3) in which a sulfur atom constituting athiocarbonyl group has a Mulliken charge of −0.220 or more.

[In the formula (3), R² to R⁵ are identical to or different from eachother, and represent an electron-withdrawing group or a hydrogen atom,and at least one of R² to R⁵ is an electron-withdrawing group.]

The Mulliken charge of the sulfur atom constituting the thiocarbonylgroup (>C═S) is preferably −0.220 or more, more preferably −0.215 ormore, and even more preferably −0.210 or more, and is preferably 0 orless, more preferably −0.01 or less, and even more preferably −0.05 orless. If the Mulliken charge is −0.220 or more, the high resilience isexerted. The Mulliken charge of the sulfur atom can be controlled by thetype or position of the substituent group. In addition, substitutingmultiple electron-withdrawing groups for the hydrogen atoms in thebenzene ring can also increase the charge of the sulfur of thethiocarbonyl group.

The Mulliken charge of the sulfur atom constituting the thiocarbonylgroup is obtained by performing structure optimization calculation andvibration number calculation using Gaussian09 (quantum chemistrycalculation program available from Gaussian Inc.) under conditions offunctional: B3LYP and basis function: 6-31G (d), and calculating theMulliken charge of the sulfur atom constituting the thiocarbonyl groupin the obtained optimized structure.

Examples of the electron-withdrawing group represented by R² to R⁵ inthe formula (3) include the electron-withdrawing groups represented byR² to R⁵ in the formula (2). The electron-withdrawing group representedby R² to R⁵ in the formula (3) is preferably one member selected fromthe group consisting of a halogen group, a perfluoroalkyl group, and apentafluorosulfanyl group. Among R² to R⁵ in the formula (3), at leastone of R³ to R⁵ is preferably an electron-withdrawing group. Herein, atleast one of R² to R⁵ means any one of R², R³, R⁴ or R⁵, any combinationof two of R², R³, R⁴, and R⁵, any combination of three of R², R³, R⁴ andR⁵, or all of R², R³, R⁴, and R⁵.

(d) The benzothiazole derivative more preferably includes a compoundrepresented by the formula (4). If the electron-withdrawing grouprepresented by R³ is at the 5-position, its electronical effect on thethiocarbonyl group is greater than that of the electron-withdrawinggroup at other substituent position.

[In the formula (4), R³ represents an electron-withdrawing group.]

Examples of the electron-withdrawing group represented by R³ in theformula (4) include the electron-withdrawing groups represented by R³ inthe formula (2). The electron-withdrawing group represented by R³ in theformula (4) is preferably one member selected from the group consistingof a halogen group, a perfluoroalkyl group, and a pentafluorosulfanylgroup.

In particular, (d) the benzothiazole derivative preferably includes acompound represented by the formula (4-1) to the formula (4-12).

(d) The benzothiazole derivative preferably has a ratio (X1/X2) of lessthan 1, wherein X1 is a bond distance between a sulfur atom and a zincatom in a compound represented by the formula (5) formed from thecompound represented by the formula (1) and/or the compound representedby the formula (2), and X2 is a bond distance between a sulfur atom anda zinc atom in zinc 2-mercaptobenzothiazole (the formula (6)).

[In the formula (5), R¹² to R¹⁵ and R²² to R²⁵ are identical to ordifferent from each other, and represent an electron-withdrawing groupor a hydrogen atom, at least one of R¹² to R¹⁵ is anelectron-withdrawing group, and at least one of R²² to R²⁵ is anelectron-withdrawing group.] Herein, at least one of R¹² to R¹⁵ meansany one of R¹², R¹³, R¹⁴ or R¹⁵, any combination of two of R¹², R¹³,R¹⁴, and R¹⁵, any combination of three of R¹², R¹³, R¹⁴ and R¹⁵, or allof R¹², R¹³, R¹⁴, and R¹⁵. At least one of R²² to R²⁵ means any one ofR²², R²³, R²⁴ or R²⁵, any combination of two of R²², R²³, R²⁴, and R²⁵,any combination of three of R²², R²³, R²⁴ and R²⁵, or all of R²², R²³,R²⁴, and R²⁵.

The ratio (X1/X2) is preferably less than 1, more preferably 0.999 orless. It is considered that (d) the benzothiazole derivative reacts withthe metal constituting the metal salt of (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms, or (e) the metal compound inthe rubber composition, to form a complex. It is considered thatformation of this complex affects the reactivity of (b) theco-crosslinking agent, and changes the crosslinking speed. It isconsidered that if the ratio (X1/X2) is less than 1, the crosslinkingspeed is further enhanced, and the obtained cured product has increasedhardness. It is noted that the lower limit of the ratio (X1/X2) is0.990.

In addition, (d) the benzothiazole derivative preferably has a ratio(Y1/Y2) of more than 1, more preferably 1.0005 or more, and even morepreferably 1.0010 or more, wherein Y1 is a bond distance between anitrogen atom and a zinc atom in the compound represented by the formula(5) formed from the compound represented by the formula (1) and/or thecompound represented by the formula (2), and Y2 is a bond distancebetween a nitrogen atom and a zinc atom in zinc 2-mercaptobenzothiazole(the formula (6)). It is noted that the upper limit of the ratio (Y1/Y2)is 1.0100.

The bond distances X1, X2, Y1 and Y2 are obtained by performingstructure optimization calculation and vibration number calculationusing Gaussian09 (quantum chemistry calculation program available fromGaussian Inc.) under conditions of functional: B3LYP and basis function:6-31G (d), and calculating the bond distance between the sulfur atom andthe zinc atom, and the bond distance between the nitrogen atom and thezinc atom in the obtained optimized structure. Examples of theelectron-withdrawing group represented by R¹² to R¹⁵ and R²² to R²⁵ inthe formula (5) include the electron-withdrawing groups represented byR² to R⁵ in the formula (1). The electron-withdrawing group representedby R¹² to R¹⁵ and R²² to R²⁵ in the formula (5) is preferably one memberselected from the group consisting of a halogen group, a perfluoroalkylgroup, and a pentafluorosulfanyl group. Among R¹² to R¹⁵ in the formula(5), at least one of R¹³ to R¹⁵ is preferably an electron-withdrawinggroup. Among R²² to R²⁵ in the formula (5), at least one of R²³ to R²⁵is preferably an electron-withdrawing group.

The amount of (d) the benzothiazole derivative is preferably 0.01 partby mass or more, more preferably 0.1 part by mass or more, and even morepreferably 0.5 part by mass or more, and is preferably 20 parts by massor less, more preferably 10 parts by mass or less, and even morepreferably 5 parts by mass or less, with respect to 100 parts by mass of(a) the base rubber. If the amount of (d) the benzothiazole derivativeis 0.01 part by mass or more, the resilience is further enhanced, and ifthe amount of (d) the benzothiazole derivative is 20 parts by mass orless, the softness is better.

In the rubber composition, the charge ratio (anion charge/cation charge)of the total charge of the anion component deriving from (d) thebenzothiazole derivative to the total charge of the cation componentderiving from (b) the co-crosslinking agent and (e) the metal compoundis preferably 0.01 or more, more preferably 0.02 or more, and ispreferably 0.10 or less, more preferably 0.05 or less. The total chargeof the anion component is equal to moles of (d) the benzothiazolederivative. The total charge of the cation component is a sum of aproduct obtained by multiplying mole of each metal ion by valence ofeach metal ion in (b) the co-crosslinking agent, and a product obtainedby multiplying mole of each metal ion by valence of each metal ion in(e) the metal compound.

Next, other materials used in the rubber composition will be explained.

(a) Base Rubber

As (a) the base rubber, a natural rubber and/or a synthetic rubber canbe used. Examples of the synthetic rubber include a diene rubber such aspolybutadiene rubber (BR), polyisoprene rubber (IR),styrene-polybutadiene rubber (SBR), chloroprene rubber (CR), butylrubber (IIR), and acrylonitrile-butadiene rubber (NBR); and a non-dienerubber such as ethylene-propylene rubber (EPM), ethylene-propylene-dienerubber (EPDM), urethane rubber, silicone rubber, acrylic rubber,epichlorohydrin rubber, polysulfide rubber, fluorinated rubber, andchlorosulfonated polyethylene rubber. The rubber may be used solely, ortwo or more of them may be used in combination.

(a) The base rubber preferably contains the natural rubber and/or thediene rubber. The total amount of the natural rubber and/or the dienerubber in (a) the base rubber is preferably 50 mass % or more, morepreferably 70 mass % or more, and even more preferably 90 mass % ormore. It is also preferable that (a) the base rubber consists of thenatural rubber and/or the diene rubber.

(a) The base rubber preferably includes a polybutadiene rubber, andparticularly preferably includes a high-cis polybutadiene having acis-1,4 bond in an amount of 40 mass % or more, preferably 80 mass % ormore, and more preferably 90 mass % or more in view of its superiorresilience. The amount of the high-cis polybutadiene in (a) the baserubber is preferably 50 mass % or more, more preferably 70 mass % ormore.

The amount of the 1,2-vinyl bond in the high-cis polybutadiene ispreferably 2.0 mass % or less, more preferably 1.7 mass % or less, andeven more preferably 1.5 mass % or less. If the amount of the 1,2-vinylbond is excessively great, the resilience may be lowered.

The high-cis polybutadiene is preferably a polybutadiene synthesizedusing a rare earth element catalyst. When a neodymium catalyst, whichemploys a neodymium compound that is a lanthanum series rare earthelement compound, is used, a polybutadiene rubber having a high contentof a cis-1,4 bond and a low content of a 1,2-vinyl bond is obtained withexcellent polymerization activity. Such a polybutadiene rubber isparticularly preferred.

The high-cis polybutadiene preferably has a molecular weightdistribution Mw/Mn (Mw: weight average molecular weight, Mn: numberaverage molecular weight) of 2.0 or more, more preferably 2.2 or more,even more preferably 2.4 or more, and most preferably 2.6 or more, andpreferably has a molecular weight distribution Mw/Mn of 6.0 or less,more preferably 5.0 or less, even more preferably 4.0 or less, and mostpreferably 3.4 or less. If the molecular weight distribution (Mw Mn) ofthe high-cis polybutadiene is excessively low, the processabilitydeteriorates. If the molecular weight distribution (Mw/Mn) of thehigh-cis polybutadiene is excessively high, the resilience may belowered. It is noted that the measurement of the molecular weightdistribution is conducted by gel permeation chromatography(“HLC-8120GPC”, available from Tosoh Corporation) using a differentialrefractometer as a detector under the conditions of column: GMHHXL(available from Tosoh Corporation), column temperature: 40° C., andmobile phase: tetrahydrofuran, and calculated by converting based onpolystyrene standard.

The Mooney viscosity (ML₁₊₄ (100° C.)) of the high-cis polybutadiene ispreferably 30 or more, more preferably 32 or more, and even morepreferably 35 or more, and is preferably 140 or less, more preferably120 or less, even more preferably 100 or less, and most preferably 80 orless. It is noted that the Mooney viscosity (ML₁₊₄ (100° C.)) in thepresent disclosure is a value measured according to JIS K6300 using an Lrotor under the conditions of: a preheating time of 1 minute; a rotorrevolution time of 4 minutes; and a temperature of 100° C.

(b) Co-Crosslinking Agent

(b) The co-crosslinking agent has an action of crosslinking a rubbermolecule by graft polymerization to a base rubber molecular chain. (b)The co-crosslinking agent preferably includes an α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof.The α,β-unsaturated carboxylic acid used as (b) the co-crosslinkingagent preferably has 3 to 8 carbon atoms, more preferably has 3 to 6carbon atoms, and even more preferably has 3 or 4 carbon atoms. It isnoted that the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms and/or the metal salt thereof may be used solely, or two or moreof them may be used in combination.

Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms include acrylic acid, methacrylic acid, fumaric acid, maleic acidand crotonic acid. When the rubber composition contains only theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as theco-crosslinking agent, the rubber composition preferably furthercontains (e) a metal compound. Neutralizing the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms with the metal compound inthe rubber composition provides substantially the same effect as usingthe metal salt of the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms as the co-crosslinking agent.

Examples of the metal ion constituting the metal salt of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include amonovalent metal ion such as sodium, potassium and lithium; a divalentmetal ion such as magnesium, calcium, zinc, barium and cadmium; atrivalent metal ion such as aluminum; and other metal ion such as tinand zirconium. The above metal component may be used solely or as amixture of at least two of them. Among them, the divalent metal ion suchas magnesium, calcium, zinc, barium and cadmium is preferably used asthe metal component. This is because if the divalent metal salt of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is used, ametal crosslinking easily generates between the rubber molecules.Especially, the divalent metal salt is preferably the zinc salt of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, morepreferably zinc acrylate, because use of such divalent metal saltenhances the resilience of the obtained golf ball. It is noted that whenthe α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is usedtogether with the metal salt thereof as the co-crosslinking agent, (e)the metal compound can be used as an optional component.

When the metal is a divalent or trivalent metal, the metal salt of (b)the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms mayfurther include another carboxylic acid than the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms, as the carboxylic acidmoiety. Examples of the other carboxylic acids include a saturatedcarboxylic acid such as caprylic acid, capric acid, lauric acid,myristic acid, palmitic acid, stearic acid, arachidic acid, and behenicacid; and an unsaturated carboxylic acid such as palmitoleic acid, oleicacid, linoleic acid, linolenic acid, and arachidonic acid.

The amount of (b) the co-crosslinking agent is preferably 15 parts bymass or more, more preferably 20 parts by mass or more, even morepreferably 25 parts by mass or more, and most preferably 27 parts bymass or more, and is preferably 50 parts by mass or less, morepreferably 45 parts by mass or less, and even more preferably 35 partsby mass or less, with respect to 100 parts by mass of (a) the baserubber. If the amount of (b) the co-crosslinking agent is 15 parts bymass or more, the member formed from the rubber composition can be madeto have an appropriate hardness with a small amount of (c) thecrosslinking initiator, and thus the crosslinked rubber molded producthas further enhanced resilience. On the other hand, if the amount of (b)the co-crosslinking agent is 50 parts by mass or less, the member formedfrom the rubber composition is not excessively hard.

(c) Crosslinking Initiator

(c) The crosslinking initiator is blended to crosslink (a) the baserubber component. As (c) the crosslinking initiator, an organic peroxideis suitable. Specific examples of the organic peroxide include dicumylperoxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy) hexane and di-t-butyl peroxide. Theseorganic peroxides may be used solely or as a mixture of at least two ofthem. Among them, dicumyl peroxide is preferably used.

The amount of (c) the crosslinking initiator is preferably 0.2 part bymass or more, more preferably 0.5 part by mass or more, and even morepreferably 0.7 part by mass or more, and is preferably 5.0 parts by massor less, more preferably 2.5 parts by mass or less, even more preferably2.0 parts by mass or less, and most preferably 0.9 part by mass or less,with respect to 100 parts by mass of (a) the base rubber. If the amountof (c) the crosslinking initiator is 0.2 part by mass or more, thecrosslinked rubber molded product formed from the rubber composition isnot excessively soft, and thus the resilience is better, and if theamount of (c) the crosslinking initiator is 5.0 parts by mass or less,the crosslinked rubber molded product formed from the rubber compositionhas an appropriate hardness, and thus the resilience and the durabilityare better.

In the rubber composition, the molar ratio ((d)/(c)) of the moles of (d)the benzothiazole derivative to the moles of (c) the crosslinkinginitiator is preferably 0.5 or more, more preferably 1.0 or more, andeven more preferably 1.5 or more, and is preferably 10 or less, morepreferably 7 or less, and even more preferably 5 or less. If the molarratio ((d)/(c)) is 0.5 or more, the obtained crosslinked rubber hasfurther enhanced hardness, and if the molar ratio ((d)/(c)) is 10 orless, the obtained crosslinked rubber has better properties.

(e) Metal Compound

When the rubber composition used in the present disclosure contains onlythe α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as theco-crosslinking agent, the rubber composition further contains (e) ametal compound as an essential component. (e) The metal compound is notparticularly limited, as long as (e) the metal compound is capable ofneutralizing (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms in the rubber composition. Examples of (e) the metalcompound include a metal hydroxide such as magnesium hydroxide, zinchydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide,potassium hydroxide, and copper hydroxide; a metal oxide such asmagnesium oxide, calcium oxide, zinc oxide, and copper oxide; and ametal carbonate such as magnesium carbonate, zinc carbonate, calciumcarbonate, sodium carbonate, lithium carbonate, and potassium carbonate.As (e) the metal compound, the divalent metal compound is preferable,the zinc compound is more preferable. This is because the divalent metalcompound reacts with the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms to form a metal crosslinking. In addition, if the zinccompound is used, the obtained crosslinked rubber molded product hashigher resilience. (e) The metal compound may be used solely, or atleast two of them may be used in combination. The amount of (e) themetal compound can be appropriately adjusted according to the desiredneutralization degree of (b) the α,β-unsaturated carboxylic acid having3 to 8 carbon atoms and (g) the unsaturated aliphatic carboxylic acid.

(f) Organic Sulfur Compound

The rubber composition may further contain (f) an organic sulfurcompound. (f) The organic sulfur compound excludes (d) the benzothiazolederivative. Examples of (f) the organic sulfur compound include at leastone member selected from the group consisting of thiophenols,thionaphthols, polysulfides, thiurams, thiocarboxylic acids,dithiocarboxylic acids, sulfenamides, dithiocarbamates, thiazoles, andmetal salts thereof. As (f) the organic sulfur compound, the organicsulfur compound having a thiol group (—SH), or the metal salt thereof ispreferable, thiophenols, thionaphthols, or the metal salt thereof aremore preferable.

Examples of the thiols include thiophenols and thionaphthols. Examplesof the thiophenols include thiophenol; thiophenols substituted with afluoro group, such as 4-fluorothiophenol, 2,5-difluorothiophenol,2,6-difluorothiophenol, 2,4,5-trifluorothiophenol,2,4,5,6-tetrafluorothiophenol and pentafluorothiophenol; thiophenolssubstituted with a chloro group, such as 2-chlorothiophenol,4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol,2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol,2,4,5,6-tetrachlorothiophenol and pentachlorothiophenol; thiophenolssubstituted with a bromo group, such as 4-bromothiophenol,2,5-dibromothiophenol, 2,6-dibromothiophenol, 2,4,5-tribromothiophenol,2,4,5,6-tetrabromothiophenol and pentabromothiophenol; thiophenolssubstituted with an iodo group, such as 4-iodothiophenol,2,5-diiodothiophenol, 2,6-diiodothiophenol, 2,4,5-triiodothiophenol,2,4,5,6-tetraiodothiophenol and pentaiodothiophenol; and metal saltsthereof. As the metal salt, zinc salt is preferable.

Examples of the thionaphthols (naphthalenethiols) include2-thionaphthol, 1-thionaphthol, 1-chloro-2-thionaphthol,2-chloro-1-thionaphthol, 1-bromo-2-thionaphthol, 2-bromo-1-thionaphthol,1-fluoro-2-thionaphthol, 2-fluoro-1-thionaphthol,1-cyano-2-thionaphthol, 2-cyano-1-thionaphthol, 1-acetyl-2-thionaphthol,2-acetyl-1-thionaphthol, and metal salts thereof. Among them,2-thionaphthol, 1-thionaphthol, and metal salts thereof are preferable.As the metal salt, a divalent metal salt is preferable, zinc salt ismore preferable. Specific examples of the metal salt include zinc saltof 1-thionaphthol and zinc salt of 2-thionaphthol.

The polysulfides are organic sulfur compounds having a polysulfide bond,and examples thereof include disulfides, trisulfides, and tetrasulfides.As the polysulfides, diphenyl polysulfides are preferable.

Examples of the diphenyl polysulfides include diphenyl disulfide;diphenyl disulfides substituted with a halogen group, such asbis(4-fluorophenyl) disulfide, bis(2,5-difluorophenyl) disulfide,bis(2,6-difluorophenyl) disulfide, bis(2,4,5-trifluorophenyl) disulfide,bis(2,4,5,6-tetrafluorophenyl) disulfide, bis(pentafluorophenyl)disulfide, bis(4-chlorophenyl) disulfide, bis(2,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl) disulfide, bis(2,4,5-trichlorophenyl)disulfide, bis(2,4,5,6-tetrachlorophenyl) disulfide,bis(pentachlorophenyl) disulfide, bis(4-bromophenyl) disulfide,bis(2,5-dibromophenyl) disulfide, bis(2,6-dibromophenyl) disulfide,bis(2,4,5-tribromophenyl) disulfide, bis(2,4,5,6-tetrabromophenyl)disulfide, bis(pentabromophenyl) disulfide, bis(4-iodophenyl) disulfide,bis(2,5-diiodophenyl) disulfide, bis(2,6-diiodophenyl) disulfide,bis(2,4,5-triiodophenyl) disulfide, bis(2,4,5,6-tetraiodophenyl)disulfide and bis(pentaiodophenyl) disulfide; and diphenyl disulfidessubstituted with an alkyl group, such as bis(4-methylphenyl) disulfide,bis(2,4,5-trimethylphenyl) disulfide, bis(pentamethylphenyl) disulfide,bis(4-t-butylphenyl) disulfide, bis(2,4,5-tri-t-butylphenyl) disulfide,and bis(penta-t-butylphenyl) disulfide.

Examples of the thiurams include thiuram monosulfides such astetramethylthiuram monosulfide; thiuram disulfides such astetramethylthiuram disulfide, tetraethylthiuram disulfide andtetrabutylthiuram disulfide; and thiuram tetrasulfides such asdipentamethylenethiuram tetrasulfide. Examples of the thiocarboxylicacids include naphthalene thiocarboxylic acid. Examples of thedithiocarboxylic acids include naphthalene dithiocarboxylic acid.Examples of the sulfenamides include N-cyclohexyl-2-benzothiazolesulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, andN-t-butyl-2-benzothiazole sulfenamide.

(f) The organic sulfur compound may be used solely or as a mixture of atleast two of them. As (f) the organic sulfur compound, the thiophenolsand/or the metal salts thereof, the thionaphthols and/or the metal saltsthereof, the diphenyl disulfides, and the thiuram disulfides arepreferable, 2,4-dichlorothiophenol, 2,6-difluorothiophenol,2,6-dichlorothiophenol, 2,6-dibromothiophenol, 2,6-diiodothiophenol,2,4,5-trichlorothiophenol, pentachlorothiophenol, 1-thionaphthol,2-thionaphthol, diphenyl disulfide, bis(2,6-difluorophenyl) disulfide,bis(2,6-dichlorophenyl) disulfide, bis(2,6-dibromophenyl) disulfide,bis(2,6-diiodophenyl) disulfide, and bis(pentabromophenyl) disulfide aremore preferable.

The amount of (f) the organic sulfur compound is preferably 0.05 part bymass or more, more preferably 0.1 part by mass or more, and ispreferably 5.0 parts by mass or less, more preferably 2.0 parts by massor less, with respect to 100 parts by mass of (a) the base rubber. Ifthe amount of (f) the organic sulfur compound is less than 0.05 part bymass, the effect of adding (f) the organic sulfur compound may not beobtained, and thus the resilience of the crosslinked rubber moldedproduct may not be enhanced. In addition, if the amount of (f) theorganic sulfur compound is more than 5.0 parts by mass, the obtainedcrosslinked rubber molded product has a great compression deformationamount and thus the resilience thereof may be lowered.

In the rubber composition, the mass ratio ((d)/(f)) of (d) thebenzothiazole derivative to (f) the organic sulfur compound ispreferably 0.5 or more, more preferably 1 or more, and even morepreferably 1.5 or more, and is preferably 10 or less, more preferably 8or less, and even more preferably 5 or less. If the mass ratio ((d)/(f)is 0.5 or more, the crosslinking can proceed more effectively, and ifthe mass ratio ((d)/(f) is 10 or less, the obtained crosslinked rubberhas better properties.

(g) Carboxylic Acid and/or Salt Thereof

The rubber composition may further contain (g) a carboxylic acid and/ora salt thereof. If the rubber composition contains (g) the carboxylicacid and/or the salt thereof, the hardness distribution of the obtainedcrosslinked rubber molded product can be controlled. Examples of (g) thecarboxylic acid and/or the salt thereof include an aliphatic carboxylicacid, an aliphatic carboxylic acid salt, an aromatic carboxylic acid andan aromatic carboxylic acid salt. (g) The carboxylic acid and/or thesalt thereof may be used solely, or as a mixture of two or more of them.It is noted that (g) the carboxylic acid and/or the salt thereofexcludes the α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand the metal salt thereof used as (b) the co-crosslinking agent.

The aliphatic carboxylic acid may be either a saturated aliphaticcarboxylic acid (hereinafter sometimes referred to as “saturated fattyacid”) or an unsaturated aliphatic carboxylic acid (hereinaftersometimes referred to as “unsaturated fatty acid”). In addition, thealiphatic carboxylic acid may have a branched or cyclic structure. Thesaturated fatty acid preferably has 6 or more carbon atoms, andpreferably has 24 or less carbon atoms, more preferably has 18 or lesscarbon atoms, and even more preferably has 13 or less carbon atoms. Theunsaturated fatty acid preferably has 6 or more carbon atoms, morepreferably has 7 or more carbon atoms, and even more preferably has 8 ormore carbon atoms, and preferably has 24 or less carbon atoms, morepreferably has 18 or less carbon atoms, and even more preferably has 13or less carbon atoms.

Examples of the aromatic carboxylic acid include a carboxylic acidhaving a benzene ring in the molecule, and a carboxylic acid having anaromatic heterocycle in the molecule. The aromatic carboxylic acid maybe used solely, or two or more of them may be used in combination.Examples of the carboxylic acid having the benzene ring include anaromatic carboxylic acid having a carboxyl group directly bonding to abenzene ring, an aromatic-aliphatic carboxylic acid having an aliphaticcarboxylic acid bonding to a benzene ring, a polynuclear aromaticcarboxylic acid having a carboxyl group directly bonding to a fusedbenzene ring, and a polynuclear aromatic-aliphatic carboxylic acidhaving an aliphatic carboxylic acid bonding to a fused benzene ring.Examples of the carboxylic acid having the aromatic heterocycle includea carboxylic acid having a carboxyl group directly bonding to anaromatic heterocycle.

As the aliphatic carboxylic acid salt or aromatic carboxylic acid salt,a salt of the above mentioned aliphatic carboxylic acid or aromaticcarboxylic acid can be used. Examples of the cation component of thesesalts include a metal ion, an ammonium ion, and an organic cation. Thecation component may be used solely, or two or more of them may be usedin combination. Examples of the metal ion include a monovalent metal ionsuch as sodium, potassium, lithium and silver; a divalent metal ion suchas magnesium, calcium, zinc, barium, cadmium, copper, cobalt, nickel andmanganese; a trivalent metal ion such as aluminum and iron; other ionsuch as tin, zirconium and titanium. Among them, the metal ion ispreferably the divalent metal ion, more preferably magnesium, zinc, orcalcium.

The organic cation is a cation having a carbon chain. The organic cationis not particularly limited, and examples thereof include an organicammonium ion. Examples of the organic ammonium ion include a primaryammonium ion such as stearyl ammonium ion, hexyl ammonium ion, octylammonium ion and 2-ethylhexyl ammonium ion; a secondary ammonium ionsuch as dodecyl(lauryl) ammonium ion and octadecyl(stearyl) ammoniumion; a tertiary ammonium ion such as trioctyl ammonium ion; and aquaternary ammonium ion such as dioctyldimethyl ammonium ion anddistearyldimethyl ammonium ion. These organic cations may be usedsolely, or two or more of them may be used in combination.

Examples of the aliphatic carboxylic acid and/or the salt thereofinclude a saturated fatty acid and/or a salt thereof, and an unsaturatedfatty acid and/or a salt thereof. The saturated fatty acid and/or thesalt thereof is preferable, and caprylic acid (octanoic acid),pelargonic acid (nonanoic acid), capric acid (decanoic acid), lauricacid, myristic acid, palmitic acid, stearic acid, behenic acid, andtheir potassium salt, magnesium salt, calcium salt, aluminum salt, zincsalt, iron salt, copper salt, nickel salt and cobalt salt, arepreferable. As the unsaturated fatty acid and/or the salt thereof,palmitoleic acid, oleic acid, linoleic acid, arachidonic acid, and theirpotassium salt, magnesium salt, calcium salt, aluminum salt, zinc salt,iron salt, copper salt, nickel salt and cobalt salt, are preferable.

As the aromatic carboxylic acid and/or the salt thereof, benzoic acid,butylbenzoic acid, anisic acid (methoxybenzoic acid), dimethoxybenzoicacid, trimethoxybenzoic acid, dimethylaminobenzoic acid, chlorobenzoicacid, dichlorobenzoic acid, trichlorobenzoic acid, acetoxybenzoic acid,biphenylcarboxylic acid, naphthalenecarboxylic acid,anthracenecarboxylic acid, furancarboxylic acid, thenoic acid, and theirpotassium salt, magnesium salt, calcium salt, aluminum salt, zinc salt,iron salt, copper salt, nickel salt and cobalt salt, are particularlypreferable.

For example, the amount of (g) the carboxylic acid and/or the saltthereof is preferably 1 part by mass or more, more preferably 2 parts bymass or more, and even more preferably 3 parts by mass or more, and ispreferably 30 parts by mass or less, more preferably 20 parts by mass orless, and even more preferably 15 parts by mass or less, with respect to100 parts by mass of (a) the base rubber.

(Other Components)

The rubber composition may contain additives such as a pigment, a fillerfor adjusting weight or the like, an antioxidant, a peptizing agent, anda softener, where necessary. In addition, the rubber composition maycontain a rubber powder obtained by pulverizing a golf ball core oroffcuts produced when preparing a core.

Examples of the pigment blended in the rubber composition include awhite pigment, a blue pigment, and a purple pigment. As the whitepigment, titanium oxide is preferably used. The type of titanium oxideis not particularly limited, but rutile type is preferably used becauseof its high opacity. In addition, the amount of titanium oxide ispreferably 0.5 part by mass or more, more preferably 2 parts by mass ormore, and is preferably 8 parts by mass or less, more preferably 5 partsby mass or less, with respect to 100 parts by mass of (a) the baserubber.

It is also preferred that the rubber composition contains both the whitepigment and the blue pigment. The blue pigment is blended in order tocause white color to be vivid, and examples thereof include ultramarineblue, cobalt blue, and phthalocyanine blue. In addition, examples of thepurple pigment include anthraquinone violet, dioxazine violet, andmethyl violet.

The filler blended in the rubber composition is used as a weightadjusting agent for adjusting the mass of the obtained crosslinkedrubber molded product. The filler may be blended where necessary.Examples of the filler include an inorganic filler such as zinc oxide,barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, andmolybdenum powder.

The amount of the antioxidant is preferably 0.1 part by mass or more and1 part by mass or less with respect to 100 parts by mass of (a) the baserubber. In addition, the amount of the peptizing agent is preferably 0.1part by mass or more and 5 parts by mass or less with respect to 100parts by mass of (a) the base rubber.

Preparation of Rubber Composition

The rubber composition used in the present disclosure can be obtained bymixing and kneading (a) the base rubber, (b) the co-crosslinking agent,(c) the crosslinking initiator, (d) the benzothiazole derivative, andother additives where necessary. The kneading method is not particularlylimited. For example, the kneading can be conducted by using aconventional kneading machine such as a kneading roll, a banbury mixer,and a kneader.

[Crosslinked Rubber Molded Product]

The crosslinked rubber molded product according to the presentdisclosure is formed from the above-described rubber composition. Thecrosslinked rubber molded product can be obtained by molding the kneadedrubber composition in a mold. The molding temperature is preferably 120°C. or more, more preferably 150° C. or more, and is preferably 250° C.or less. In addition, the molding pressure preferably ranges 2.9 MPa to11.8 MPa. The molding time preferably ranges from 10 minutes to 60minutes.

Examples of the crosslinked rubber molded product include a sports goodssuch as a golf ball, a tennis ball and a grip; an industrial goods suchas a hose, a belt, and a mat; a sole, a tire, a resin additive, ananti-vibration rubber, and a fender. Examples of the golf ball include agolf ball comprising a constituent member formed from theabove-described rubber composition.

Examples

Next, the present disclosure will be described in detail by way ofexamples. However, the present disclosure is not limited to the examplesdescribed below. Various changes and modifications without departingfrom the spirit of the present disclosure are included in the scope ofthe present disclosure.

[Evaluation Method] (1) Compression Deformation Amount (Mm)

The deformation amount of the spherical molded product along thecompression direction (shrinking amount of the spherical molded productalong the compression direction), when applying a load from 98 N as aninitial load to 1275 N as a final load to the spherical molded product,was measured.

(2) Coefficient of Restitution

A metal cylindrical object with a mass of 198.4 g was allowed to collidewith each spherical molded product at a speed of 40 m/sec, and thespeeds of the cylindrical object and the spherical molded product beforeand after the collision were measured. Based on the speed and the massof each object, the coefficient of restitution of each spherical moldedproduct was calculated. The measurement was conducted by using twelvesamples for each spherical molded product, and the average value thereofwas adopted as the coefficient of restitution of the spherical moldedproduct.

(3) Slab Hardness (Shore C)

Sheets with a thickness of about 2 mm were produced by heat-pressmolding at the temperature of 170° C. for 20 minutes. The sheets werestored at a temperature of 23±2° C. and a relative humidity of 50%±5%for at least 12 hours. At least three of these sheets were stacked onone another so as not to be affected by the measuring substrate on whichthe sheets were placed, and the hardness of the stack was measured withan automatic hardness tester (Digitest II, available from Bareisscompany) using a testing device of “Shore C”.

[Production of Spherical Molded Product]

The rubber compositions having the formulations shown in Table 1 werekneaded with a kneading roll, and heat pressed at a temperature of 170°C. for 20 minutes in upper and lower molds, each having a hemisphericalcavity, to obtain spherical molded products having a diameter of 40.86mm.

TABLE 1 Spherical molded product No. 1 2 3 4 5 6 7 8 9 Formulation BR100 100 100 100 100 100 100 100 100 (parts by ZN-DA90S 25 28 25 28 25 2825 28 25 mass) ZnO 5 5 5 5 5 5 5 5 5 PCTPZn 0.5 0.5 0.5 0.5 0.5 0.5 0.50.5 0.5 5Me-MBT — — — — — — — — — 4Me-MBT — — — — — — — — — MBT — — — —— — — — — 5Cl-MBT 1.21 1.21 — — — — — — — 5F-MBT — — 1.11 1.11 — — — — —5CF₃-MBT — — — — 1.41 1.41 — — — 5Br-MBT — — — — — — 1.47 1.47 — 6F-MBT— — — — — — — — 1.21 DCP 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Mullikencharge of sulfur atom −0.206 −0.206 −0.211 −0.211 −0.202 −0.202 −0.206−0.206 −0.214 constituting thiocarbonyl group Properties of Mass (g)34.7 34.9 34.7 34.9 34.7 34.9 34.7 34.9 35.1 core Compression 3.8 3.43.8 3.4 3.8 3.4 3.9 3.5 3.9 deformation amount (mm) Coefficient of 0.8010.810 0.803 0.812 0.805 0.815 0.802 0.811 0.801 restitution Sphericalmolded product No. 10 11 12 13 14 15 16 17 18 Formulation BR 100 100 100100 100 100 100 100 100 (parts by ZN-DA90S 28 25 28 25 28 23 26 25 28mass) ZnO 5 5 5 5 5 5 5 5 5 PCTPZn 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.55Me-MBT — 1.08 1.08 — — — — — — 4Me-MBT — — — 1.08 1.08 — — — — MBT — —— — — 1.00 1.00 — — 5Cl-MBT — — — — — — — — — 5F-MBT — — — — — — — — —5CF₃-MBT — — — — — — — — — 5Br-MBT — — — — — — — — — 6F-MBT 1.21 — — — —— — — — DCP 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Mulliken charge ofsulfur atom −0.214 −0.224 −0.224 −0.227 −0.227 −0.221 −0.221 — —constituting thiocarbonyl group Properties of Mass (g) 35.4 34.6 34.834.7 34.9 35.0 35.2 34.6 35.1 core Compression 3.4 4.2 3.8 4.8 4.4 3.43.0 4.2 3.5 deformation amount (mm) Coefficient of 0.808 0.790 0.7990.779 0.786 0.810 0.817 0.793 0.806 restitution

BR: “BR730” (high-cis polybutadiene rubber (cis-1,4 bond content=96 mass%, 1,2-vinyl bond content=1.3 mass %, Moony viscosity (ML₁₊₄ (100°C.)=55, molecular weight distribution (Mw/Mn)=3)) available from JSRCorporation

ZN-DA90S: zinc acrylate (including 10 mass % of zinc stearate) availablefrom Nisshoku Techno Fine Chemical Co., Ltd.

ZnO: “Ginrei R” (zinc oxide) available from Toho Zinc Co., Ltd.

PCTP-Zn: pentachlorothiophenol zinc salt available from FUJIFILM WakoChemicals Corporation

5Me-MBT: 5-methyl-2-mercaptobenzothiazole available from Cool pharm Ltd.

4Me-MBT: 4-methyl-2-mercaptobenzothiazole available from Fluorochem Ltd.

MBT: 2-mercaptobenzothiazole (NOCCELER M-P) available from Ouchi ShinkoChemical Industrial Co., Ltd.

5Cl-MBT: 5-chloro-2-mercaptobenzothiazole available from Tokyo Chemicalindustry Co., Ltd.

5F-MBT: 5-fluoro-2-mercaptobenzothiazole available from Combi-BlocksInc.

5CF₃-MBT: 5-trifluoromethyl-2-mercaptobenzothiazole available from AABlocks Inc.

5Br-MBT: 5-bromo-2-mercaptobenzothiazole available from Sigma-AldrichCo. LLC.

6F-MBT: 6-fluoro-2-mercaptobenzothiazole available from BLD PharmatechInc.

DCP: “Percumyl (register trademark) D” (dicumyl peroxide) available fromNOF Corporation

Table 1 shows the compression deformation amount and the coefficient ofrestitution of each spherical molded product. In addition, the FIGUREshows the relationship between the compression deformation amount andthe coefficient of restitution of each spherical molded product. Asshown in the FIGURE, there is a tendency that the amount of theco-crosslinking agent is greater, the resilience performance is higher,and the compression deformation amount is smaller, when the materialscontained in the rubber composition are same. Thus, it can be said thatthe spherical molded product formed from the same materials has highersoftness and more excellent resilience performance, if the lineconnecting the plots of the spherical molded product is located in theupper right of the graph.

The spherical molded products No. 1 to 10 are formed from the rubbercomposition containing (e) the metal compound, and the compoundrepresented by the formula (1) and/or the compound represented by theformula (2) as (d) the benzothiazole derivative. Specifically, thespherical molded products No. 1 to 10 are formed from the rubbercomposition containing the benzothiazole derivative which is a compoundrepresented by the formula (3) in which a sulfur atom constituting athiocarbonyl group has a Mulliken charge of −0.220 or more. Thesespherical molded products have higher softness and more excellentresilience performance than the spherical molded products No. 17, 18 notcontaining the benzothiazole derivative.

[Preparation of Rubber Composition]

The rubber compositions having the formulations shown in Tables 2 and 3were kneaded with a kneading roll to prepare the rubber compositions.The obtained rubber compositions were evaluated.

TABLE 2 Rubber composition No. 19 20 21 22 23 24 Formulation BR 100 100100 100 100 100 (parts by ZN-DA90S 25 28 25 28 25 28 mass) ZnO 5 5 5 5 55 PCTPZn 0.5 0.5 0.5 0.5 0.5 0.5 5Cl-MBT 1.21 1.21 — — — — 5F-MBT — —1.08 1.08 — — MBT — — — — — — 5CF₃-MBT — — — — 1.21 1.21 6F-MBT — — — —— — 4Me-MBT — — — — — — 5Me-MBT — — — — — — 4Cl-MBT — — — — — — DCP 0.80.8 0.8 0.8 0.8 0.8 Charge ratio 0.02 0.02 0.02 0.02 0.02 0.01 (anioncharge/ cation charge) Bond Distance 2.4032 2.4032 2.4049 2.4049 2.40292.4029 distance between S—Zn (Å) Ratio 0.9982 0.9982 0.9989 0.99890.9981 0.9981 (X1/X2) Distance 2.0398 2.0398 2.0380 2.0380 2.0406 2.0406between N—Zn (Å) Ratio 1.0016 1.0016 1.0007 1.0007 1.0020 1.0020 (Y1/Y2)Properties Hardness 78.5 83.3 79.0 82.0 77.2 81.6 of slab (Shore C)Rubber composition No. 25 26 27 28 29 30 Formulation BR 100 100 100 100100 100 (parts by ZN-DA90S 25 28 26 28 29 32 mass) ZnO 5 5 5 5 5 5PCTPZn 0.5 0.5 0.5 0.5 0.5 0.5 5Cl-MBT — — — — — — 5F-MBT — — — — — —MBT — — — — — — 5CF₃-MBT — — — — — — 6F-MBT 1.21 1.21 — — — — 4Me-MBT —— — — — — 5Me-MBT — — — — — — 4Cl-MBT — — — — — — DCP 0.8 0.8 0.8 0.80.8 0.8 Charge ratio 0.02 0.02 — — — — (anion charge/ cation charge)Bond Distance 2.4037 2.4037 — — — — distance between S—Zn (Å) Ratio0.9984 0.9984 — — — — (X1/X2) Distance 2.0400 2.0400 — — — — betweenN—Zn (Å) Ratio 1.0017 1.0017 — — — — (Y1/Y2) Properties Hardness 77.881.7 77.6 79.9 81.0 83.1 of slab (Shore C)

TABLE 3 Rubber composition No. 31 32 33 34 35 36 37 38 39 40 FormulationBR 100 100 100 100 100 100 100 100 100 100 (parts by ZN-DA90S 23 26 2829 25 28 25 28 25 28 mass) ZnO 5 5 5 5 5 5 5 5 5 5 PCTPZn 0.5 0.5 0.50.5 0.5 0.5 0.5 0.5 0.5 0.5 5Cl-MBT — — — — — — — — — — 5F-MBT — — — — —— — — — — MBT 1.0 1.0 1.0 1.0 — — — — — — 5CF₃-MBT — — — — — — — — — —6F-MBT — — — — — — — — — — 4Me-MBT — — — — 1.08 1.08 — — — — 5Me-MBT — —— — — — 1.08 1.08 — — 4Cl-MBT — — — — — — — — 1.21 1.21 DCP 0.8 0.8 0.80.8 0.8 0.8 0.8 0.8 0.8 0.8 Charge ratio 0.02 0.02 0.02 0.02 0.02 0.020.02 0.02 0.02 0.02 (anion charge/cation charge) Bond Distance 2.40752.4075 2.4075 2.4075 2.4023 2.4023 2.4084 2.4084 2.4222 2.4222 distancebetween S—Zn (Å) Ratio (X1/X2) 1.0000 1.0000 1.0000 1.0000 0.9978 0.99781.0004 1.0004 1.0061 1.0061 Distance 2.0365 2.0365 2.0365 2.0365 2.03952.0395 2.0355 2.0355 2.0333 2.0333 between N—Zn (Å) Ratio (Y1/Y2) 1.00001.0000 1.0000 1.0000 1.0015 1.0015 0.9995 0.9995 0.9984 0.9984Properties Hardness 75.7 79.3 81.3 82.3 63.6 67.3 76.5 80.2 67.5 71.7 ofslab (Shore C)

BR: “BR730” (high-cis polybutadiene rubber (cis-1,4 bond content=96 mass%, 1,2-vinyl bond content=1.3 mass %, Moony viscosity (ML₁₊₄ (100°C.)=55, molecular weight distribution (Mw/Mn)=3)) available from JSRCorporation

ZN-DA90S: zinc acrylate (including 10 mass % of zinc stearate) availablefrom Nisshoku Techno Fine Chemical Co., Ltd.

ZnO: “Ginrei R” (zinc oxide) available from Toho Zinc Co., Ltd.

PCTP-Zn: pentachlorothiophenol zinc salt available from FUJIFILM WakoChemicals Corporation

5Cl-MBT: 5-chloro-2-mercaptobenzothiazole available from Tokyo Chemicalindustry Co., Ltd.

5F-MBT: 5-fluoro-2-mercaptobenzothiazole available from Combi-BlocksInc.

MBT: 2-mercaptobenzothiazole (NOCCELER M-P) available from Ouchi ShinkoChemical Industrial Co., Ltd.

5CF₃-MBT: 5-trifluoromethyl-2-mercaptobenzothiazole available from AABlocks Inc.

6F-MBT: 6-fluoro-2-mercaptobenzothiazole available from BLD PharmatechInc.

4Me-MBT: 4-methyl-2-mercaptobenzothiazole available from Fluorochem Ltd.

5Me-MBT: 5-methyl-2-mercaptobenzothiazole available from Cool pharm Ltd.

4Cl-MBT: 4-chloro-2-mercaptobenzothiazole available from Fluorochem Ltd.

DCP: “Percumyl (register trademark) D” (dicumyl peroxide) available fromNOF Corporation

The spherical molded products No. 19 to 26 are formed from the rubbercomposition containing (e) the metal compound, and the compoundrepresented by the formula (1) and/or the compound represented by theformula (2) as (d) the benzothiazole derivative. Specifically, therubber compositions No. 19 to 26 are the cases containing a compoundwhich is the compound represented by the formula (5) having a ratio(X1/X2) of less than 1, as (d) the benzothiazole derivative.

The rubber compositions No. 27 to 30 are the cases not containing (d)the benzothiazole derivative. The rubber compositions No. 31 to 34 arethe cases containing the mercaptobenzothiazole not having a substituentgroup. By comparing No. 20, 22, 24, 26, 28 and 33 having the same amountof the co-crosslinking agent among these rubber compositions, it isfound that No. 20, 22, 24, 26 have higher slab hardness than No. 33.

The rubber compositions No. 35 to 40 are the cases containing thebenzothiazole derivative not having an electron-withdrawing group at5-position to 7-position. By comparing No. 33, 36, 38 and 40 having thesame amount of the co-crosslinking agent among these rubbercompositions, it is found that No. 36, 38, 40 have lower slab hardnessthan No. 33. It is noted that although 4Me-MBT has a ratio (X1/X2) ofless than 1, the hardness improvement effect was not obtained becausethe substituent group is not an electron-withdrawing group.

If the rubber composition according to the present disclosure is used, acrosslinked rubber molded product having excellent resilienceperformance is obtained. Thus, the rubber composition according to thepresent disclosure can be used in sports goods such as a golf ball, atennis ball and a grip; industrial goods such as a hose, a belt, and amat; a sole, a tire, a resin additive, an anti-vibration rubber, and afender, and so on.

The present disclosure (1) is directed to a rubber compositioncontaining (a) a base rubber, (b) an α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms and/or a metal salt thereof as aco-crosslinking agent, (c) a crosslinking initiator, and (d) abenzothiazole derivative, and further comprising (e) a metal compoundwhen (b) the co-crosslinking agent consists of the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms, wherein (d) thebenzothiazole derivative includes a compound represented by the formula(1) and/or a compound represented by the formula (2).

[R¹ represents a hydrogen atom, an alkyl group having 1 to 8 carbonatoms, an aryl group having 4 to 14 carbon atoms, or a metal atom,

R² to R⁵ are identical to or different from each other, and represent anelectron-withdrawing group or a hydrogen atom, and at least one of R³ toR⁵ is an electron-withdrawing group.]

The present disclosure (2) is directed to the rubber compositionaccording to the present disclosure (1), wherein theelectron-withdrawing group represented by R² to R⁵ is one memberselected from the group consisting of a halogen group, a perfluoroalkylgroup, and a pentafluorosulfanyl group.

The present disclosure (3) is directed to the rubber compositionaccording to the present disclosure (1) or (2), wherein an amount of (d)the benzothiazole derivative ranges from 0.01 part by mass to 20 partsby mass with respect to 100 parts by mass of (a) the base rubber.

The present disclosure (4) is directed to the rubber compositionaccording to any one of the present disclosure (1) to (3), wherein (d)the benzothiazole derivative includes a compound represented by theformula (3) in which a sulfur atom constituting a thiocarbonyl group hasa Mulliken charge of −0.220 or more.

[In the formula (3), R² to R⁵ are identical to or different from eachother, and represent an electron-withdrawing group or a hydrogen atom,and at least one of R² to R⁵ is an electron-withdrawing group.]

The present disclosure (5) is directed to the rubber compositionaccording to the present disclosure (4), wherein (d) the benzothiazolederivative includes a compound represented by the formula (4).

[In the formula (4), R³ represents an electron-withdrawing group.]

The present disclosure (6) is directed to the rubber compositionaccording to any one of the present disclosure (1) to (3), wherein aratio (X1/X2) is less than 1, wherein X1 is a bond distance between asulfur atom and a zinc atom in a compound represented by the formula (5)formed from the compound represented by the formula (1) and/or thecompound represented by the formula (2), and X2 is a bond distancebetween a sulfur atom and a zinc atom in zinc 2-mercaptobenzothiazole.

[In the formula (5), R¹² to R¹⁵ and R²² to R²⁵ are identical to ordifferent from each other, and represent an electron-withdrawing groupor a hydrogen atom, at least one of R¹² to R¹⁵ is anelectron-withdrawing group, and at least one of R²² to R²⁵ is anelectron-withdrawing group.]

The present disclosure (7) is directed to the rubber compositionaccording to any one of the present disclosure (1) to (6), wherein anamount of (b) the co-crosslinking agent ranges from 15 parts by mass to50 parts by mass with respect to 100 parts by mass of (a) the baserubber.

The present disclosure (8) is directed to a crosslinked rubber moldedproduct formed from the rubber composition according to any one of thepresent disclosure (1) to (7).

The present disclosure (9) is directed to a golf ball comprising aconstituent member formed from the rubber composition according to anyone of the present disclosure (1) to (7).

This application is based on Japanese patent applications No.2021-098142 filed on Jun. 11, 2021 and No. 2021-180405 filed on Nov. 4,2021, the contents of which are hereby incorporated by reference.

1. A rubber composition containing (a) a base rubber, (b) anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof as a co-crosslinking agent, (c) a crosslinkinginitiator, and (d) a benzothiazole derivative, and further comprising(e) a metal compound when (b) the co-crosslinking agent consists of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, wherein (d)the benzothiazole derivative includes a compound represented by theformula (1) and/or a compound represented by the formula (2),

R¹ represents a hydrogen atom, an alkyl group having 1 to 8 carbonatoms, an aryl group having 4 to 14 carbon atoms, or a metal atom, R² toR⁵ are identical to or different from each other, and represent anelectron-withdrawing group or a hydrogen atom, and at least one of R³ toR⁵ is an electron-withdrawing group.
 2. The rubber composition accordingto claim 1, wherein the electron-withdrawing group represented by R² toR⁵ is one member selected from the group consisting of a halogen group,a perfluoroalkyl group, and a pentafluorosulfanyl group.
 3. The rubbercomposition according to claim 1, wherein an amount of (d) thebenzothiazole derivative ranges from 0.01 part by mass to 20 parts bymass with respect to 100 parts by mass of (a) the base rubber.
 4. Therubber composition according to claim 1, wherein (d) the benzothiazolederivative includes a compound represented by the formula (3) in which asulfur atom constituting a thiocarbonyl group has a Mulliken charge of−0.220 or more,

wherein in the formula (3), R² to R⁵ are identical to or different fromeach other, and represent an electron-withdrawing group or a hydrogenatom, and at least one of R² to R⁵ is an electron-withdrawing group. 5.The rubber composition according to claim 4, wherein (d) thebenzothiazole derivative includes a compound represented by the formula(4),

wherein in the formula (4), R³ represents an electron-withdrawing group.6. The rubber composition according to claim 1, wherein a ratio (X1/X2)is less than 1, wherein X1 is a bond distance between a sulfur atom anda zinc atom in a compound represented by the formula (5) formed from thecompound represented by the formula (1) and/or the compound representedby the formula (2), and X2 is a bond distance between a sulfur atom anda zinc atom in zinc 2-mercaptobenzothiazole,

wherein in the formula (5), R¹² to R¹⁵ and R²² to R²⁵ are identical toor different from each other, and represent an electron-withdrawinggroup or a hydrogen atom, at least one of R¹² to R¹⁵ is anelectron-withdrawing group, and at least one of R²² to R²⁵ is anelectron-withdrawing group.
 7. The rubber composition according to claim1, wherein an amount of (b) the co-crosslinking agent ranges from 15parts by mass to 50 parts by mass with respect to 100 parts by mass of(a) the base rubber.
 8. A crosslinked rubber molded product formed froma rubber composition containing (a) a base rubber, (b) anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof as a co-crosslinking agent, (c) a crosslinkinginitiator, and (d) a benzothiazole derivative, and further comprising(e) a metal compound when (b) the co-crosslinking agent consists of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, wherein (d)the benzothiazole derivative includes a compound represented by theformula (1) and/or a compound represented by the formula (2),

R¹ represents a hydrogen atom, an alkyl group having 1 to 8 carbonatoms, an aryl group having 4 to 14 carbon atoms, or a metal atom, R² toR⁵ are identical to or different from each other, and represent anelectron-withdrawing group or a hydrogen atom, and at least one of R³ toR⁵ is an electron-withdrawing group.
 9. The crosslinked rubber moldedproduct according to claim 8, wherein the electron-withdrawing grouprepresented by R² to R⁵ is one member selected from the group consistingof a halogen group, a perfluoroalkyl group, and a pentafluorosulfanylgroup.
 10. The crosslinked rubber molded product according to claim 8,wherein an amount of (d) the benzothiazole derivative ranges from 0.01part by mass to 20 parts by mass with respect to 100 parts by mass of(a) the base rubber.
 11. The crosslinked rubber molded product accordingto claim 8, wherein (d) the benzothiazole derivative includes a compoundrepresented by the formula (3) in which a sulfur atom constituting athiocarbonyl group has a Mulliken charge of −0.220 or more,

wherein in the formula (3), R² to R⁵ are identical to or different fromeach other, and represent an electron-withdrawing group or a hydrogenatom, and at least one of R² to R⁵ is an electron-withdrawing group. 12.The crosslinked rubber molded product according to claim 11, wherein (d)the benzothiazole derivative includes a compound represented by theformula (4),

wherein in the formula (4), R³ represents an electron-withdrawing group.13. The crosslinked rubber molded product according to claim 8, whereina ratio (X1/X2) is less than 1, wherein X1 is a bond distance between asulfur atom and a zinc atom in a compound represented by the formula (5)formed from the compound represented by the formula (1) and/or thecompound represented by the formula (2), and X2 is a bond distancebetween a sulfur atom and a zinc atom in zinc 2-mercaptobenzothiazole,

wherein in the formula (5), R¹² to R¹⁵ and R²² to R²⁵ are identical toor different from each other, and represent an electron-withdrawinggroup or a hydrogen atom, at least one of R¹² to R¹⁵ is anelectron-withdrawing group, and at least one of R²² to R²⁵ is anelectron-withdrawing group.
 14. The crosslinked rubber molded productaccording to claim 8, wherein an amount of (b) the co-crosslinking agentranges from 15 parts by mass to 50 parts by mass with respect to 100parts by mass of (a) the base rubber.
 15. A golf ball comprising aconstituent member formed from a rubber composition containing (a) abase rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms and/or a metal salt thereof as a co-crosslinking agent, (c) acrosslinking initiator, and (d) a benzothiazole derivative, and furthercomprising (e) a metal compound when (b) the co-crosslinking agentconsists of the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms, wherein (d) the benzothiazole derivative includes a compoundrepresented by the formula (1) and/or a compound represented by theformula (2),

R¹ represents a hydrogen atom, an alkyl group having 1 to 8 carbonatoms, an aryl group having 4 to 14 carbon atoms, or a metal atom, R² toR⁵ are identical to or different from each other, and represent anelectron-withdrawing group or a hydrogen atom, and at least one of R³ toR⁵ is an electron-withdrawing group.
 16. The golf ball according toclaim 15, wherein the electron-withdrawing group represented by R² to R⁵is one member selected from the group consisting of a halogen group, aperfluoroalkyl group, and a pentafluorosulfanyl group.
 17. The golf ballaccording to claim 15, wherein an amount of (d) the benzothiazolederivative ranges from 0.01 part by mass to 20 parts by mass withrespect to 100 parts by mass of (a) the base rubber.
 18. The golf ballaccording to claim 15, wherein (d) the benzothiazole derivative includesa compound represented by the formula (3) in which a sulfur atomconstituting a thiocarbonyl group has a Mulliken charge of −0.220 ormore,

wherein in the formula (3), R² to R⁵ are identical to or different fromeach other, and represent an electron-withdrawing group or a hydrogenatom, and at least one of R² to R⁵ is an electron-withdrawing group. 19.The golf ball according to claim 18, wherein (d) the benzothiazolederivative includes a compound represented by the formula (4),

wherein in the formula (4), R³ represents an electron-withdrawing group.20. The golf ball according to claim 15, wherein a ratio (X1/X2) is lessthan 1, wherein X1 is a bond distance between a sulfur atom and a zincatom in a compound represented by the formula (5) formed from thecompound represented by the formula (1) and/or the compound representedby the formula (2), and X2 is a bond distance between a sulfur atom anda zinc atom in zinc 2-mercaptobenzothiazole,

wherein in the formula (5), R¹² to R¹⁵ and R²² to R²⁵ are identical toor different from each other, and represent an electron-withdrawinggroup or a hydrogen atom, at least one of R¹² to R¹⁵ is anelectron-withdrawing group, and at least one of R²² to R²⁵ is anelectron-withdrawing group.
 21. The golf ball according to claim 15,wherein an amount of (b) the co-crosslinking agent ranges from 15 partsby mass to 50 parts by mass with respect to 100 parts by mass of (a) thebase rubber.